Phosphorus is one of the most important nutrients for plants. It is essential for their growth and is a structural component of nucleic acids, phospholipids, intermediary metabolites and numerous other biological molecules.
In plants, the only readily absorbed form of exogenous phosphorus is inorganic phosphate (P.sub.i) (Bieleski, 1973). When the amount of available phosphate is low, plants are unable to grow vigorously and productively. When phosphate is absent, growth is halted and the plant dies.
Because they are sessile organisms, plants must deal biochemically with environmental stresses such as temperature extremes, nutrient deficiency and drought. This is also true for other photosynthetic organisms which are either sessile or limited in movement. Plants and other photosynthetic organisms, therefore, require signal transduction pathways in order to trigger cellular responses to adverse environmental stimuli.
It has long been known that both temporal and quantitative characteristics of flowering are affected by the level of phosphate in plants relative to the level of nitrogen (Salisbury and Ross, 1985). Relatively high phosphate advances maturity in plants, whereas relatively low phosphate results in little or no flowering taking place. Phosphate levels are also known to affect the biomass ratio between root and shoot. Specifically, phosphate deprivation causes preferential growth of roots (Lefebvre et al., 1982). Thus, in many environments, the availability of phosphorus becomes a major factor limiting the growth and reproduction of photosynthetic organisms.
Numerous groups have investigated the nature of the phosphate-starvation response in plants but despite these studies, little is known of the molecular mechanisms that regulate phosphorus uptake and metabolism. In general, plants exhibit significant morphological and physiological changes in response to perturbations within the environment.
There have been many attempts to identify proteins which are induced under conditions of phosphate starvation. Fife et al. (1990) have conducted in vivo protein labeling studies in Brassica nigra cells grown in suspension in either rich or low phosphate medium. Using 2-dimensional gel electrophoresis, they demonstrated the novel synthesis of four proteins under P.sub.i deficiency and one protein in well-nourished cells. Other groups have reported that P.sub.i deprivation increases the synthesis of a plasma membrane protein and a soluble protein in tomato root cultures (Hawkesford and Belcher, 1991), and enhances secretion of six proteins from tomato suspension cells (Goldstein et al., 1989). It has also been shown that a gene for a protein homologous to .beta.-glucosidases is induced to high levels in B. nigra suspension cells under P.sub.i starvation (Malboobi and Lefebvre, 1995).
As part of the adenosine nucleotides, ADP and ATP, which are the currency of cellular energy, phosphorus is critical to bioenergetics. Further, the covalent addition or removal of a phosphate group to or from a biological substrate (phosphorylation and dephosphorylation, respectively) often functions as a kind of regulatory "on/off switch" in cellular metabolism and signal transduction. For example, the phosphorylation and dephosphorylation of certain membrane-bound receptor protein kinases and their substrates are key to various signal transduction pathways, including pathways of plant hormones such as ethylene (Kieber et al., 1993) and abscisic acid (Anderberg and Walker-Simmons, 1992). Self-incompatiblity with respect to pollination and fertilization also involves the activity of protein kinases encoded by S-locus genes (Tantikanjana et al., 1993; Zhang and Walker, 1993).
Knowledge of the proteins which affect the uptake and accumulation of phosphorus and which are expressed in phosphate-deficient environments is essential to understand phosphate metabolism and to manipulate the growth and reproduction of photosynthetic organisms for commercial or industrial purposes. Further, the identification and synthesis of the genes which encode such proteins would allow the development of transgenic photosynthetic organisms for many purposes.